Aeronautical engine technology is developing rapidly, and one of the avenues explored for improving the specific consumption of civil airplane engines is currently that of developing open-rotor engines. Engines of this kind, such as that described in the patent application FR 2 941 493 by the applicant company, comprise a conventional turboshaft engine gas generator, one or more turbine stages of which drive an unducted fan which extends outside the engine. For reasons of the thrust level to be reached and of reducing the noise generated by the blades, the engine generally comprises two contra-rotating unducted propellers, i.e. propellers which rotate in opposite directions and which can be positioned at the front or at the rear of the engine, according to the configuration selected, for example in order to position the propellers as far as possible from the passenger cabin.
The general configuration of an open-rotor engine is shown in FIG. 1.
As in the case of conventional turboprop engines, the propeller blades of the open-rotor engines have a variable pitch, i.e. the pitch of these propellers can be modified during flight in order to change the thrust of the engine and optimize the output of the propeller in accordance with the speed of the aircraft. A number of devices have been conceived in order to vary the pitch of the blades, which devices generally comprise setting the vane into rotation about the main axis thereof by means of conical pinions located below the root of the vane. The pinions engage with conical pinions of a control system.
In normal use, during flight phases, the pitch of a propeller changes between two limit boundaries which correspond to a low-pitch position at low travel speeds, which is of approximately 30° relative to the plane of rotation of the propellers, and a high-pitch position at high speeds, which is of approximately 65° relative to this same plane of rotation of the propellers. The feathered position corresponds to a pitch which is greater than that of the high-pitch position and is equal to approximately 90°. The pitches in these normal use conditions are conventionally known as positive.
One of the limitations of systems for controlling the pitch of propeller blades is having to bring the blades into the position known as “feathered” in the event of the pitching system failing. The feathered position consists in rotating the propeller blades until the chord thereof is substantially aligned into the wind, thus reducing the drag generated by the blades and, consequently, the yaw imbalance produced on the airplane. Another intention is to reduce the residual rotational speed of the propellers.
The feathered position must correspond to a position of equilibrium which is automatically assumed by the propeller blades when the system for controlling the pitch no longer transmits any torque.
Conventionally, the return to the feathered position is ensured by what are known as direct counterweights, the inertia of which, being far greater than that of the blades, should ensure that the blades are returned to the feathered position defined above.
According to another solution described in the patent application WO 2012/066240, filed by the applicant, balance weights which form counterweights are attached to the conical pinions of the system for controlling the pitch of the blades and are arranged having an overhang relative thereto. The system is integrated in the spaces between the blades in order to have a minimal axial and/or radial size. In normal operation, the blades are held in position by means of the control system. In the event of the system failing, the action of the centrifugal force due to the rotation of the propeller brings the blades towards a rest position which corresponds to the feathered position of the blade. According to the embodiment mentioned in this application, the conical pinion of the blade root causes a reduction ratio of approximately 2 (two) between the pitch angle of the counterweight and the pitch angle of the blade. Other embodiments relating to devices for feathering blades of a propeller, comprising balance weights which drive a pinion that is rigidly connected to the root of the blades, are described in GB 2 218 747, FR 2 864 942, GB 124 935 or U.S. Pat. No. 2,353,334.
These two embodiments each have a disadvantage. The solution comprising a counterweight which is positioned directly on the axis of rotation is disadvantageous in terms of weight, because it requires the use of a heavy balance weight, the effect of which is not multiplied. The solution comprising a balance weight which meshes directly with the conical pinion for controlling the rotation of the blade makes it possible to reduce the weight used on account of the multiplication, but the freedom of choice is still limited due to restrictions relating to the size of the device relative to the space available. These limitations may prevent integration in the plane of the propeller.